Developers used in electrophotographic systems such as copying machine include two-component developer, which comprises carrier and toner, and single component developer, which is free of carrier. Conventional toners used in both types of developers consist of fine particles of binder resin in which colorants, such as pigment and dye, are dispersed.
One of the important properties of the toner is triboelectric chargeability. That is, properties required for toners include that they can generate a suitable amount of positive or negative electrostatic charge upon contacting with a carrier or a charge providing device, and that the amount of the charge is stable over continuous use or under an adverse environment. Binder resin, the main component of the toner, may be triboelectrically chargeable but the amount of the generated electrostatic charge is not sufficient, and therefore image fogging may occur to make the resulting image unclear. In order to impart further triboelectric charging property, toners in general are added with a charge control agent so that the electrostatic charge is controlled suitably.
Generally known and used charge control agent for electrophotographic toners include positively chargeable agents such as nigrosin dyes and quaternary ammonium salts, and negatively chargeable agents such as metal containing monoazo dyes, metal-salicylic acid complexs and copper phthalocyanine pigments.